Attention, or the ability to select information for further processing, is critical for survival. When the ability to focus attention is disrupted, such as in attention deficit disorder or spatial neglect, the decrease in quality of life can be devastating. There are several brain areas in which damage or identifiable activity changes correlate with attentional dysfunction. These include the parahippocampal cortex, called the postrhinal cortex (POR) in rodents, and the pulvinar nucleus of the thalamus, sometimes called the lateral posterior nucleus in the rodent brain. Disruption of these regions is implicated in neuropsychiatric conditions, including schizophrenia and depression, as well as attention-related deficits, including attention deficit disorder, Parkinson?s disease, and Alzheimer?s disease. Anatomical work confirms that the pulvinar and the POR are robustly and reciprocally connected. The pulvinar provides, by far, the strongest subcortical input to the POR, and the POR provides more input to the caudomedial region of the pulvinar than any other neocortical structure. Whereas these two regions have been studied individually in the context of attention, the role of the POR-pulvinar circuit in attention has not been investigated. The PI proposes to use in vivo electrophysiology, computational modeling, and optogenetics to investigate the organization and function of the POR-pulvinar circuit. Aim 1 will consist of simultaneously recording single unit and local field potential data from the POR and the pulvinar using tetrode recording methods while rats perform a visuospatial attention task. The PI will analyze both single unit and field potential data for correlations between the two regions with a focus on field potential oscillations in the theta (6-10 Hz) and fast gamma (65-100 Hz) frequency bands. In aim 2, the PI will use the data collected in Aim 1 to fit a General Linear Model network that accounts for observed correlated activity and performance on the visuospatial attention task. This model will be used to explore how the POR- pulvinar circuit functions and how disruptions in the circuit might affect behavior. In aim 3, the PI will use optogenetic inhibition of the POR-projecting pulvinar neurons, as well as the pulvinar-projecting POR neurons, to test the predictions of the model and the involvement of the POR-pulvinar circuit in the task. In addition to elucidating the circuits underlying attention, these studies will provide new information about the organization and function of thalamocortical and corticothalamic circuits in the brain. This research will inform future research aimed at understanding and treating attentional dysfunction.